1. Field of the Invention
The present invention relates to an image-taking lens system and an image-taking system, such as an image-taking lens which is favorable for television cameras or video cameras, for example, and in particular to an image-taking lens system and an image-taking system which are favorable for a plurality of image-taking apparatuses with different image sizes or an image-taking apparatus which can switch between a plurality of image sizes, and which are suitable for auto-wobbling to search the in-focus point by wobbling a portion of a lens unit arranged further to the image side than a light amount adjustment-stop.
2. Background of the Invention
In recent years, as camera operators and image-taking conditions for image-taking lenses for television recordings diversify, the requirements regarding operability, convenience and maneuverability of such lenses are becoming more stringent. Accordingly, the importance of image-taking lenses having an automatic focusing adjustment (AF, autofocus) function is increasing.
Ordinarily, due to the need to ensure the operability and maneuverability of image-taking lenses such as lenses for TV cameras, it is not appropriate to use an autofocus of the external measurement method using infrared light or the like, and which has to be provided with a distance detecting system which is separate from the image-forming system.
In video cameras or the like, the so-called “hill-climbing AF” method is common, in which the direction approaching the best image-forming position is detected from the video signal, and a focus lens unit is moved in this direction up to a suitable position.
FIG. 3 shows the optical structure of a zoom lens having such a hill-climbing AF function. In FIG. 3, I denotes a first lens unit which can be moved in an optical axis direction for performing focusing, II denotes a second lens unit which can be moved during zooming, III denotes a third lens unit for correcting the movement of the image-forming position during zooming, SP denotes a light amount adjustment stop, IV denotes a fourth lens unit performing an image-forming action, GB denotes a glass block such as a color separation optical system, Wo denotes a wobbling lens unit, and IG denotes an image-taking surface (CCD surface or film surface).
In order to determine with this setup whether the best image-forming position is located frontward or rearward in the optical axis direction with respect to the image-taking surface, the video signal is detected while a portion of the optical system is finely driven by an amplitude (wobbling) in the optical axis direction, and based on this signal, the focus lens unit is moved such that the optimum focus is attained on the image-taking surface. In order to allow driving of the wobbling lens unit with a small driving force and to simplify its structure, it is common to make the lens diameters of the wobbling lens unit small, and to perform the wobbling with a portion of a lens unit which is further on the image side than the light amount adjustment stop, which is fixed during zooming. On the one hand, the image-forming position should be shifted with a suitable amplitude that is possible to determine a direction as the system and a peak in video signal, in order to detect gradients in the video signal more accurately, and the amplitude should be restricted, so that the image on the output screen does not become blurry due to the amplitude driving of the wobbling lens units.
Now, there are pluralities of image sizes in TV cameras, and various kinds of camera bodies are arranged in accordance with the individual image sizes. Also, image-taking lenses need to be designed exclusively in accordance with each image size and camera body. When an image-taking lens for large image sizes is used on a camera body having a small image-size, then it is not possible to obtain the same field angle, and the focal length becomes too longer than a diagonal length in the image size. That is to say, the focal length will be substantially shifted to the telephoto side.
On the other hand, there are camera systems which can be switched between screen aspect ratios of 16:9 and 4:3 by making the left and right portions of screens with a screen aspect ratio of 16:9 into imperceptibility portion (by cutting the left and right portions of screens) to attain a screen aspect ratio of 12:9=4:3. When switching from 16:9 to 4:3, the image-taking field angle is narrowed due to the reduction of the image size. In order to solve this problem, for example Japanese Patent No. 2840283 proposes an image-taking lens system, in which a favorable optical performance is maintained in spite of large changes in the field angle by mounting an auxiliary lens system which can be inserted into the optical system in order to shift the focal length to the wide-angle side, for example, when the image-taking lens mounted to a camera body having a large image size is remounted to a camera body having a small image size.
Incidentally, even though the lens system of Japanese Patent No. 2840283, in which a portion of a relay lens unit whose function is an image-forming action and which is arranged further on the image side than the light amount adjustment stop is wobbled, is characterized in that it achieves a smaller size and a lighter weight, inserting and removing an auxiliary lens system restricts the axial rays with a member different from the light amount adjustment stop, so that the effective F number is different even for the same diameters of the light amount adjustment stop. That is to say, if an index of the F number is marked on the ring of the light amount adjustment stop, or if the F number is calculated using a signal from a position detection unit such as a potentiometer detecting the diameter of the light amount adjustment stop, then the effective F number will be different from the indicated F number or the F number value calculated from the signal. Due to this change in the effective F number, the depth of focus changes, so that it becomes difficult to detect the best image-forming position accurately with the same amplitude of the wobbling lens unit when the auxiliary lens system is removed or when it is inserted, or there is the problem that the image on the output screen becomes blurry.